In audio applications, a power amplifier is usually incorporated to drive a speaker load. The speaker load value typically varies from 4 ohm to 32 ohm. Traditionally, class AB amplifiers have been used to drive the speaker load. A class AB amplifier includes a differential amplifier, a push-pull output stage, and a class AB bias circuit. The push-pull output stage includes a p-type transistor and an n-type transistor that drive a signal to an output. Further, the class AB bias circuit is configured to bias the push-pull output stage.
The main drawback of using class AB amplifiers is low efficiency. Low efficiency translates into more power dissipation inside the chip which in turn mandates using a heat sink to dissipate the heat and also lowers the battery life. This increases the cost and the area of the audio solution.
A class-D amplifier is attractive because of the higher power efficiency compared to a class AB amplifier, which realizes a longer battery life and eliminates the heat sink requirement. This allows design of compact and low cost multi-channel high-power systems. The class D amplifiers partially mitigate the concern of thermal runaway by using transistors as switches.
Class D amplifiers typically employ either pulse width modulation or sigma-delta modulation. The high frequency carrier signal (e.g. a triangle or a pulse waveform) is modulated by the audio input signal. This modulated signal is applied to the speaker by power transistors which are either completely on or completely off. This way the power dissipation is minimal inside these power transistors.